1. Field of the Invention
This invention relates to refractory based materials, particularly to tungsten powders and refractory metal based composite particles.
2. Description of the Related Art
Two of the more common refractory metal based bulk composites include molybdenum and tungsten. These bulk composite materials of Mo, W or compounds of these metals are strengthened by the presence of other compounds (such as carbides, borides and nitrides). These bulk composites possess a unique combination of characteristics such as high strength, toughness, electrical and thermal conductivity, tailored thermal expansion coefficient, excellent high temperature strength and good wear/corrosion/erosion resistance. They have a wide range of applications in high-temperature high-strength services in advanced nuclear reactors, power plants, rocket propulsion and space power systems, superconducting devices, radiation shielding (from gamma rays and hard x-rays), vibration damping devices, heavy duty electrical contacts, thermal management substrates, a variety of relays, high voltage switches and oil cooled circuit breakers, counterweights, boring bars, semiconductor substrates, rotational gyroscope members in inertial guidance systems, shaped charge liner materials, and other military applications such as kinetic energy projectiles.
A number of disclosures of Mo and W based bulk composites, such as WC--Co, Mo--Cu, W--Cu, Mo--Ag, W--Ag, Mo--Ni, W--Ni, W--Cu--Ni, W--Fe--Ni, and W--Ni--Co exist in the literature. As used herein, a bulk composite is a mixture or mechanical combination on a macro scale of two or more materials that are solid in the finished state, are mutually insoluble, and differ in chemical nature. Various processes have been used for making these materials, including powder metallurgical conversion, infiltration, liquid phase sintering, mechanical alloying, fluidized bed CVD coating processes, thermo-chemical processes, and spray conversion processes.
A conventional powder metallurgical conversion process used to produce WC--Co includes the following steps:
a) reducing tungsten oxide, tungstic acid, ammonium meta-tungstate or ammonium para-tungstate to tungsten metal with hydrogen; PA1 b) mixing the metallic tungsten powder and carbon black; PA1 c) heating mixed tungsten and carbon in vacuum at 1350-1600.degree. C. to form coarse WC powder; PA1 d) refining the coarse WC powder, by such means as ball milling, high energy vibratory milling or attrition milling, to form a refined WC powder with desired particle size distribution; PA1 e) coating the refined WC with Co by ball milling WC with coarse metallic cobalt. PA1 1. Preparing a starting solution of a soluble cobalt salt and a tungsten containing salt. PA1 2. Spray drying the starting solution into dry powder PA1 3. First reacting the dried powder with CO, CO/CO.sub.2 or CH.sub.4 /H.sub.2 gas mixtures that have a carbon activity equal to or greater than 1.0, in a fluidized bed reactor.
Since the process described above involves different stages such as blending, solid state reaction, crushing and grinding, the production cycle is long, the capital investment in machinery is high, and the final powder normally has a diameter above 2 to 3 microns. Various problems are encountered both in the WC synthesis and the mixing with Co. Specifically, kinetic limitations in the WC synthesis require processing at high temperature for long periods of time. In addition, compositional control is impaired by the introduction of impurities during the mechanical processing of the composite powders, primarily during the required milling operation. Moreover, the long time necessary for achieving the desired microstructure and homogenization during milling adds significantly to the overall processing costs. Finally, a uniform distribution of WC in the Co matrix is difficult to achieve.
There are several other methods for making tungsten or molybdenum bulk composites: infiltration, mechanical alloying and liquid phase sintering. The infiltration technique includes pre-sintering tungsten or molybdenum powder into a porous skeleton followed by infiltrating the skeleton with molten metallic copper, silver, gold or nickel to produce bulk composites of W--Cu, W--Ag, W--Au, W--Ni, Mo--Cu, Mo--Ag, Mo--Au and Mo--Ni.
The mechanical alloying technique involves ball milling and consolidating a mixture of tungsten (or molybdenum) powder and other ductile metallic powders (such as copper, silver, gold and nickel). The mechanically alloyed W--Cu composite, for example, normally exhibits a microstructure consisting of heavily cold-worked W platelets separated by thin Cu films.
The liquid phase sintering technique consists of first cold pressing mixture of tungsten (or molybdenum) powder and other ductile metallic powders (such as copper, silver, gold and nickel) into a pre-compact. The cold pressed compact is then heat treated to temperatures above the melting point of the ductile metal phase. During the process, rearrangement of tungsten (or molybdenum) particles occurs in the liquid phase matrix. Between tungsten and copper, there is no mutual solubility in either the solid or liquid state. For making high tungsten content (W above 80 wt %) W--Cu composite, a small amount (less than 1 wt %) of Co, Fe or Ni, is frequently added to enhance the sinterability of the W--Cu system. In W--Ni, W--Fe--Ni and W--Ni--Co systems, in contrast to the pure W--Cu system, tungsten partially dissolves in the matrix during liquid phase sintering. The process may be divided into three stages: the rearrangement, solution-reprecipitation and solid-state controlled stages. The resulting bulk composite alloys normally consist of nearly pure tungsten grains dispersed in a ductile matrix.
The biggest drawback of the conventional synthesis approaches (infiltration, mechanical alloying and liquid phase sintering) for making W- or Mo-composites is the lack of compositional homogeneity in the end products. Segregation of ductile metal phase (Cu, Ag, Au and Ni) and W or Mo phase is routinely observed, resulting in inadequate local homogeneity of thermal and electrical conductivities. Furthermore, these composites exhibit porosity and impurity effects that create deleterious effects on the thermal/electrical conductivity and the machinability of the final sintered products.
In contrast to a bulk composite, which may be made by consolidating discreet matrix material powders and strengthening phase powders, a composite particle or powder is a particle of at least two materials, each insoluble in the other. Composite particles or powders may exist as multiple component particles adhered to each other (typically, one material will be present as several smaller particles adhering to a much larger particle of the other material), discreet, intermixed phases, or a coating of a first material on a second material core. W--Ni, W--Fe--Ni and W--Ni--Co composite powders have been produced by a chemical vapor deposition process in which the tungsten powder was suspended within a gas stream containing precursors of iron, nickel and/or cobalt inside a fluidized bed reactor. The gaseous precursors decomposed inside the reaction chamber with subsequent condensation from the vapor state to form a deposit of Fe, Ni and/or Co on the individual W particle surfaces. These composite powders may then be consolidated by known powder metallurgy techniques to form bulk composite materials.
The fluidized bed-CVD approach can produce composite powders with well coated individual particles capable of being processed into final products with high compositional homogeneity. However, the scale up of such fluidized bed-CVD process is difficult. In addition, it is difficult to process sub-micron size W or Mo powders by this technology.
U.S. Pat. No. 5,441,553 discloses a thermo-chemical process for producing WC--Co powders. The process includes first synthesizing Co(en).sub.3 WO.sub.4 (en=ethylenediamene), a compound of defined composition with Co:W atomic ratio of 1:1, followed by reductive decomposition of Co(en).sub.3 WO.sub.4 powder under hydrogen at about 650.degree. C. to form W--Co powder. The W--Co powder is finally converted to WC--Co by reacting the powder with a CO/CO.sub.2 gas mixture at temperatures between 700-1000.degree. C. This process, however, is limited in its ability to produce WC--Co composite materials of varying compositions.
U.S. Pat. Nos. 3,488,291 and 5,352,269 disclose a spray conversion process which can produce WC--Co powder with 3-23 % by weight cobalt. The typical process includes the following steps.
Then removing extra free carbon deposit by gasification using CO, CO/CO.sub.2 or CH.sub.4 /H.sub.2 gas mixtures that have a carbon activity less than 1.0.
The powders produced by this spray conversion process have diameters between 10 to 50 microns, and have much smaller crystallites in the submicron (.about.0.1 .mu.m) size range.